Add missing \end, noted by George Yoshida

Doc/whatsnew/whatsnew25.tex

@@ -194,6 +194,7 @@ Traceback (most recent call last):
   File ``t.py'', line 15, in ?
     print it.next()
 StopIteration
+\end{verbatim}
 
 Because \keyword{yield} will often be returning \constant{None}, 
 you shouldn't just use its value in expressions unless you're sure